1. Cancer Research Institute, University of California at San Francisco, 2340 Sutter Street, San Francisco, California 94143, USA
2. CRC Department of Medical Oncology, University of Glasgow, Garscube Estate, Switchback Road, Glasgow G61 1BD, UK
3. Department of Dermatology, Yale University School of Medicine, New Haven, Connecticut 06520, USA
4. Department of Biochemistry and Biophysics, University of California at San Francisco, 2340 Sutter Street, San Francisco, California 94143
5. Present address: CXR Biosciences Ltd, James Lindsay Place, Dundee Technopole, Dundee DD1 5JJ, UK.

Correspondence to: Allan Balmain^1,4 Correspondence and requests for materials should be addressed to A.B. (Email: abalmain@cc.ucsf.edu).

Abstract

Mice of the C57BL/6 strain are resistant to the development of skin squamous carcinomas (SCCs) induced by an activated Ras oncogene, whereas FVB/N mice are highly susceptible¹. The genetic basis of this difference in phenotype is unknown. Here we show that susceptibility to SCC is under the control of a carboxy-terminal polymorphism in the mouse Ptch gene. F₁ hybrids between C57BL/6 and FVB/N strains ((B6FVB)F₁) are resistant to Ras-induced SCCs, but resistance can be overcome either by elimination of the C57BL/6 Ptch allele (Ptch^B6) or by overexpression of the FVB/N Ptch allele (Ptch^FVB) in the epidermis of K5Hras-transgenic (B6FVB)F₁ hybrid mice. The human Patched (PTCH) gene is a classical tumour suppressor gene for basal cell carcinomas and medulloblastomas, the loss of which causes increased signalling through the Sonic Hedgehog (SHH) pathway^{2, 3, 4, 5}. SCCs that develop in Ptch^B6+/- mice do not lose the wild-type Ptch gene or show evidence of increased SHH signalling. Although Ptch^FVB overexpression can promote SCC formation, continued expression is not required for tumour maintenance, suggesting a role at an early stage of tumour cell lineage commitment. The Ptch polymorphism affects Hras-induced apoptosis, and binding to Tid1, the mouse homologue of the Drosophila l(2)tid tumour suppressor gene. We propose that Ptch occupies a critical niche in determining basal or squamous cell lineage, and that both tumour types can arise from the same target cell depending on carcinogen exposure and host genetic background.

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